Aqueous viscosity modifiers (also referred to as “thickeners”) are ordinarily used in fields such as coating materials, adhesives, foodstuffs, cosmetic products and the like. By being added to various products, viscosity modifiers increase product viscosity, and make it possible to impart thereby various functions to the product. For instance, viscosity is ordinarily modified (for instance, thickening) using a viscosity modifier in oil-in-water emulsified compositions that are used in emulsions of pharmaceuticals, quasi-drugs or cosmetics. That is because modifying the viscosity elicits various effects such as improved skin application and creating a particular feel. The degree of viscosity modification varies significantly depending on the product. In some instances thickening is substantial, as in creams, while in other instances there is little change in viscosity, for instance in lotions.
Examples of generally known viscosity modifiers include, for instance, natural viscosity modifiers such as carboxymethyl cellulose, hydroxyethyl cellulose and the like; alkali thickening-type viscosity modifiers in which thickening is accomplished by an alkali, for instance polyacrylic acid or a polyacrylic acid-containing copolymer; and urethane viscosity modifiers such as urethane-modified polyethers and the like. Among the foregoing, numerous kinds of urethane viscosity modifiers are produced and used on account of various reasons. For instance, urethane viscosity modifiers allow producing various types of viscosity modifier more freely than is the case in other viscosity modifiers, and allow imparting various degrees of viscosity to products to which the viscosity modifier is added. When the product having the viscosity modifier added thereto is made into a coating film, urethane viscosity modifiers afford better water resistance in the coating film, as compared with that afforded by other viscosity modifiers, and are not readily influenced by pH (see, for instance, Patent Documents 1 to 3).
When formulated in, for instance, coating materials, existing urethane viscosity modifiers are however problematic on account of the occurrence of settling of solid components, such as pigments, contained in the coating material, due to long-term storage. The occurrence of separation of emulsion components due to long-term storage, when the viscosity modifiers are formulated in emulsions of cosmetic products, is a further problem. Such phenomena are observed also in natural viscosity modifiers such as carboxymethyl cellulose and hydroxyethyl cellulose, but are virtually not observed in alkali thickening-type viscosity modifiers, since the latter have good long-term storage stability.
In many instances, powders such as silica, titanium or the like are generally incorporated into oil-in-water emulsified compositions, in cosmetics or the like. When such powders are added to an oil-in-water emulsified composition that is thickened using a natural viscosity modifier or a urethane viscosity modifier, the powder separates immediately through precipitation or the like. Accordingly, alkali thickening-type viscosity modifiers having good stability after thickening (product stability) are generally used in order to thicken such oil-in-water emulsified compositions containing a powder.
As pointed out above, alkali thickening-type viscosity modifiers are mainly used in cosmetics and the like. However, alkali thickening-type viscosity modifiers have the drawback of being difficult to use, since they are significantly influenced by the pH of the system and by addition of salts into the system. In actual gel-like oil-in-water emulsified compositions that are obtained by using alkali thickening-type viscosity modifiers, the gel collapses easily due to changes in pH or addition of salts. A problem arises therefore in that components that can be added to gel-like oil-in-water emulsified compositions are limited, and thus alkali thickening-type viscosity modifiers cannot be used in a wide variety of applications.
Oil-in-water emulsified compositions are ordinarily produced through emulsification of water and oil-soluble components, using emulsifiers. Gel-like oil-in-water emulsified compositions that are produced using alkali thickening-type viscosity modifiers can only be produced by using surfactants (for instance, anionic surfactants or nonionic surfactants) as emulsifiers.
The use of surfactants is however undesirable in cosmetics (for instance, lotions), which must be hypoallergenic and safe towards the skin. Accordingly, many cosmetics are marketed in which no surfactant is utilized and in which the aqueous phase and the oil phase are in a separated state.